Wax motors provide a simple and reliable linear actuator used in a variety of applications including home appliances. Typically, a wax motor provides a metal housing holding wax and a piston so that expansion of the wax drives the piston outward. The expansion of wax occurs when the housing is heated, for example, by electrical resistance such as a positive temperature coefficient thermistor. Typically, a return spring is present in the mechanism associated with the wax motor to return the piston into the housing when the wax cools. Such wax motors develop large forces over a short stroke distance such as one-quarter inch and may be operated for many heating and cooling cycles.
The design and construction of a wax motor is described in U.S. Pat. No. 5,572,869 assigned to the assignee of the present invention and hereby incorporated by reference.
The wax motor piston may be a smooth cylindrical rod passing out of the housing through a seal that retains the wax in the housing. A common seal includes an elastomeric O-ring supported at its outer face by one or more washers, for example, a Teflon washer followed by a brass washer. The outer washer maybe held in place by a crimping of the housing around the outer washer.
The O-ring may fit within a counter bore in the housing and allows it to float in the manner of a hydraulic seal effectively preventing the escape of the thermally expansive wax.
The seal formed by the O-ring operates to reliably prevent leakage of the wax motor during initial operation. Over time with extended use, the O-ring may begin to wear at its interface with the piston shaft ultimately leading to leakage and failure of the wax motor.